It is well known that by contacting a semiconductor material with an electrolyte, an electrode is formed that is in many respects similar to the metal/electrolyte systems. In a suitable electrolyte, by applying anodic potential to the semiconductor/electrolyte electrode, the semiconductor material can be dissolved. For n-type semiconductors, the rate of anodic dissolution is determined by the amount of minority carriers created by illuminating the material. For a p-type semiconductor, high dissolution rates are obtainable without illumination. The electrochemical dissolution can be maintained at a desired rate by control of either the anodic potential or the anodic current.
The semiconductor-to-electrolyte interface is continuously renewed during this dissolution process and, under certain conditions, can be used for the determination of semiconductor characteristics that vary with the depth in the semiconductor. Such characteristics are, for example, the carrier concentration, or the layer thicknesses in multi-layer semiconductor structures, and the measured electrical parameter usually is either the capacitance or the conductance of the depletion layer of the Schottky diode formed at the semiconductor/electrolyte interface.
A method for anodic dissolution of semiconductors and simultaneous determination of their carrier concentration as a function of depth, is described by Ambridge, T. and Factor, M. M. in J. Appl. Electrochem. 319 (1975) and by Factor et al., in Current Topics in Materials Science Vol. 6 E. Kaldis, Ed., North-Holland Publ., Amsterdam, 1980. Essentially the same technique is disclosed in the U.S. Pat. Nos. 4,028,207 and 4,168,212. The semiconductor material is dissolved at a fixed potential relative to a calomel reference electrode, and the anodic current is measured between a graphite electrode and the semiconductor as the working electrode. The anodic (dissolution) potential is adjusted to a value providing a sufficiently high anodic current and resultant dissolution rate allowing the determination of the carrier concentration as a function of depth within a reasonable period of time (usually about several hours).
A method and apparatus for determining the layer thickness in multi-layer semiconductor structures by measuring the conductance of the semiconductor/electrolyte interface simultaneously with anodic etching of the semiconductor is disclosed in the U.S. patent application Ser. No. 301,889.
The contents of the U.S. Pat. Nos. 4,028,207 and 4,168,212 and U.S. Ser. No. 301,889 are hereby expressly incorporated by reference.
To ensure that a Schottky barrier is formed at the semiconductor/electrolyte interface, and to allow the reproducible measurement of the desired electrical parameters, the dissolution rate must be the same for the entire surface of the semiconductor, and the formation of any by-product on the semiconductor surface must be avoided.
The quality of the surface obtained by anodic etching of the semiconductor is greatly dependent on the electrolyte composition, as reported by G. E. Cabanies [MRS Spring Meeting 1989, Poster D5.17]. Depending on the chemical composition of the semiconductor material, hydrochloric acid, potassium or sodium hydroxide, Tiron.RTM. (a mixture of sodium fluoride and sulfuric acid), alkaline solutions of disodium ethylenediamine tetraacetate, etc. are most commonly used. The methods known in the art for the anodic dissolution of semiconductor materials try to minimize the irregularities of the semiconductor/electrolyte interface by modification of the electrolyte composition.
However, even if utmost care is taken when choosing the electrolyte composition, the semiconductor/electrolyte interface is often not reproducible and as a result, the dissolution depth calculated from the Faraday equation (this is the basis for the determination of the progress of the electrochemical dissolution) and the measurable depth will be different. A reason for this might be that the dissolution does not take place according to the expected stoichiometry and/or is accompanied by side reactions. The products of such side reactions may be deposited on the semiconductor surface interfering with the reproducibility of the anodic dissolution.